
CHAPTER 10 8-BIT A/D CONVERTER (
PD789407A SUBSERIES)
User’s Manual U13952EJ3V1UD
151
Figure 10-10. Analog Input Pin Processing
Reference voltage
input
C = 100 to 1000 pF
If noise greater than AVREF or less than
AVSS is likely to come to the AVREF pin, clamp
the voltage at the pin by attaching a diode with a
small VF (0.3 V or lower).
AVSS
AVDD
VDD0
VSS0
AVREF
(7)
ANI0 to ANI6
The analog input pins (ANI0 to ANI6) are alternate-function pins. They are also used as port pins (P60 to
P66).
If any of ANI0 to ANI6 has been selected for A/D conversion, do not execute input instructions for the ports.
Otherwise, the conversion resolution may become lower.
If a digital pulse is applied to a pin adjacent to the analog input pins during A/D conversion, coupling noise
may occur which prevents an A/D conversion result from being attained as expected. Avoid applying a digital
pulse to pins adjacent to the analog input pins during A/D conversion.
(8)
Input impedance of ANI0 to ANI6 pins
This A/D converter charges the internal sampling capacitor for about 1/10 of the conversion time, and
performs sampling.
Therefore at times other than sampling, only the leak current is output. During sampling, the current for
charging the capacitor is also output, so the input impedance fluctuates and has no meaning.
However, to ensure adequate sampling, it is recommended that the output impedance of the analog input
source be set to below 10 k
, or a 100 pF capacitor be connected to the ANI0 to ANI6 pins (see Figure 10-
10).
(9)
Input impedance of the AVREF pin
A series resistor string of several tens of k
 is connected across the AVREF and AVSS pins.
If the output impedance of the reference voltage source is high, this high impedance is eventually connected
in parallel with the series resistor string across the AVREF and AVSS pins, leading to a higher reference
voltage error.